Inhibitors of estrogen biosynthesis constitute a new therapeutic concept for the treatment of estrogen dependent diseases, in particular estrogen dependent tumors such as breast cancer. By means of the inhibition of aromatase, which is the enzyme which catalyses the last synthetic step, a selective inhibition of estrogen biosynthesis is possible. Aminoglutethimide (3-[4-aminophenyl]-3-ethyl-piperidine-2,6-dione) has been the only non-steroidal aromatase inhibitor which had been used therapeutically.